The present invention provides an improved fluid loss additive comprising a lignite grafted with at least one vinyl monomer selected from the group consisting of: dimethylaminopropyl methacrylamide, methacrylamido propyltrimethyl ammonium chloride, N-vinylformamide, N-vinylacetamide, diallyl dimethyl ammonium chloride, diallylacetamide, and diallylamine. The vinyl monomer may also be blended with at least one co-monomer. The co-monomer is preferably selected from the group consisting of: 2-acrylamido-2-methylpropanesulfonic acid, dimethylacrylamide, acrylamide, vinylpyrrolidone, vinylacetate, acrylonitrile, dimethylaminoethyl methacrylate, styrenesulfonate, vinylsulfonate, dimethylaminoethyl methacrylate methyl chloride quaternary, acrylic acid and its salts.
Fluid loss additives are typically used in drilling and cementing oil wells. That is, for oil field drilling and cementing operations to be successful, additives which reduce fluid loss are required to be added to the mud and cement. Such additives will be used in well operations where the bottom hole circulating temperature (BHCT) may range from 80-400.degree. F., substantial salt concentrations may be present, and cement slurry retardation and viscosity are critical aspects as the same effect pumpability and compressive strength. The fluid loss additive is also effective in controlling filtration properties in both fresh and sea water muds with high concentrations of soluble calcium.
Certain polymer compositions have long been recognized by those skilled in the art as additives useful in reducing fluid loss. These compositions are commonly referred to as "fluid loss additives".
Some examples of fluid loss additives are set forth in the following U.S. Pat. Nos.: 4,678,591 (Giddings et al.), issued July 7, 1987, 4,515,635 (Rao et al.), issued May 7, 1985, and 4,676,317 (Fry et al.), issued June 30, 1987.
The Rao patent discloses typical polymers used as fluid loss additives, e.g., copolymers of N,N-dimethylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid having molar ratios of between 1:4 and 4:1, respectively, and an average molecular weight of between about 75,000 and about 300,000. Other fluid loss additives disclosed in the Rao patent are: a copolymer of a sulfonic acid modified acrylamide and a polyvinyl crosslinking agent, and a hydrolyzed copolymer of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
Giddings et al. disclose a terpolymer composition for aqueous drilling fluids comprising sodium AMPS (72-3.8%), N,N-dimethylacrylamide (13.5-0.7%), and acrylonitrile (9.5-0.5%). This terpolymer composition also contains lignin, modified lignin, brown coal, or modified brown coal in an amount ranging from between 5-95% with the lignin, modified lignin, brown coal, or modified brown coal having been present during the polymerization of the water-soluble polymer. It has a molecular weight in the range of 10,000-500,000. The lignin, modified lignin, brown coal, or modified brown coal is from the group consisting of lignites, sulfonated lignites, lignins, leonardites, lignosulfonates, alkali metal humic acid salts, humic acids, and sulfonated humic acids.
Another fluid loss additive disclosed in Giddings et al. is RESINEX, a sulfonated lignite complexed with sulfonated phenolic resin.
The Fry patent discloses a graft polymer fluid loss additive comprising a backbone of at least one member selected from the group consisting of lignin, lignite, and there salts, and a grafted pendant group of at least one member selected from the group consisting of AMPS, acrylonitrile, N,N-methylacrylamide, acrylic acid, N,N-dialkylaminoethyl methacrylate, wherein the alkyl radical comprises at least one member selected from the group consisting of methyl, ethyl and propyl radicals.
The petroleum industry prefers a fluid loss additive that has as little effect on compressive strength, set time, viscosity, and thickening time as possible; a fluid loss additive that is salt tolerable (i.e., does not exhibit substantial loss of effectiveness in the presence of salt); and a fluid loss additive that is chemically stable during cementing operations. Furthermore, fluid loss additives should be compatible with as many other additives and environmental conditions as possible, should be soluble in cement slurries at normal ambient temperatures encountered in oil well cementing operations, and should continue to provide fluid loss characteristics over broad temperature and cement pH ranges.
The present inventors have developed novel vinyl grafted lignite fluid loss additives which encompass much of the aforementioned desired characteristics. That is, the novel fluid loss additives of the present invention are relatively unaffected by changes in salinity and temperature. Furthermore, the fluid loss additives of the present invention demonstrated better fluid loss characteristics in both fresh water and salt water, at varying temperatures, verses conventional fluid loss additives. The fluid loss additives of the present invention are especially effective compared to conventional fluid loss additives, e.g., Resinex, in drilling fluids at temperatures of about 350.degree. F.
The fluid loss additives of the present invention contain amine based and/or quaternary nitrogen containing monomers which are relatively hydrolytically stable in use and are salt/brine tolerant. That is, the vinyl monomers of the present invention will impart an amine or quaternary nitrogen functionality to the co-graft, such that they are stable or relatively stable under use conditions. Various vinyl monomers of the present invention will hydrolyze in a cement slurry to an amine functionality that stays with the co-graft.
Additional advantages of the present invention shall become apparent as described below.